You do not need permission from Catalyst to run Missions at conventions. All you need are willing GMs, access to some Missions adventures, some table space, and some players to show up with D6's in hand!
If you are interested in GMing some Missions adventures, or know some folks who are, contact the Convention support to see what you need to do to register events. If not, you can try contacting them anyways and see if they have GMs willing to take "requests", though often Conventions rely on Volunteer GMs (Who usually run events in exchange for a free weekend badge or something).
If you do manage to find folks interested in running them but don't have access to Missions, we can provide some, and depending on how much the group running things is doing, we can also possibly provide some kind of prize support as well. Head to
http://www.shadowrun4.com/missions/index.shtml and drop us an email through there, and we'll see what kind fo support we can supply.
I would also suggest hitting the
CGL Demo Team Site?, and either registering (If you're interested in running events) or hitting the forums and look for GMs. You can also ask in the Missions forum here on Dumpshock, as some of the Missions folks may not check in this forum.
Good luck!